Boost: Use Headers instead of building when not required

When building USD without Python, OIIO or VDB, Boost libs aren't required.
In this scenario, it is preferable to just copy over the headers instead and save some time and complexity.

This also has a significant benefit that it allows the core USD build to be supported on platforms that Boost doesn't natively build on yet. It means that the build_usd.py script doesn't need to carry as many patches to Boost itself if someone is just building the core USD library.

This is similar to [PR 2914](#2914) but can short circuit the entire bootstrap and b2 process, which may still error on platforms that b2 doesn't understand.

Author: dgovil

build_scripts/build_usd.py

@@ -756,8 +756,15 @@ def InstallBoost_Helper(context, force, buildArgs):
         "*/libs/wave/test/testwave/testfiles/utf8-test-*"
     ]
 
+    headersOnly = not (context.buildOIIO or context.enableOpenVDB or context.buildPython)
+
     with CurrentWorkingDirectory(DownloadURL(BOOST_URL, context, force, 
                                              dontExtract=dontExtract)):
+        if headersOnly:
+            headersDir = os.path.abspath("./boost")
+            CopyDirectory(context, headersDir, "include/boost")
+            return
+
         if Windows():
             bootstrap = "bootstrap.bat"
         else: